Pathfinder Armor Rules and Armor Class
Armor Class is the single number that determines whether an attack hits a character, and understanding how it's calculated can mean the difference between a fighter who shrugs off goblin arrows and one who keeps dying to them. This page covers how Armor Class is defined and calculated in Pathfinder (both first and second edition), how armor types and modifiers interact, and the practical decision points that come up most often during character creation and play.
Definition and scope
In Pathfinder, Armor Class (AC) represents how difficult a character or creature is to strike with an attack. A successful attack roll must meet or exceed the target's AC to deal damage. The system appears in both Pathfinder First Edition (released by Paizo Publishing in 2009 as a revision of Dungeons & Dragons 3.5) and Pathfinder Second Edition (released 2019), though the two editions calculate AC differently enough that they're worth treating as separate systems.
Both editions are published by Paizo Publishing, and the official rules text — the Pathfinder Core Rulebook for each edition — is the authoritative source for all calculations. The rules are also reproduced on the Archives of Nethys, the officially licensed Pathfinder rules reference, which hosts the complete rules text for free.
How it works
Pathfinder First Edition calculates AC using the following formula:
AC = 10 + Armor Bonus + Shield Bonus + Dexterity Modifier + Size Modifier + Natural Armor + Deflection Bonus + Miscellaneous Bonuses
A character wearing a breastplate (armor bonus +6) with a +2 Dexterity modifier and no other bonuses has an AC of 18. Flat-footed AC drops the Dexterity modifier, and touch AC removes all armor and shield bonuses — meaning a touch attack against that same character hits on a 12 or higher, while a normal attack needs a 19.
Pathfinder Second Edition streamlines this considerably:
AC = 10 + Level + Proficiency Bonus + Armor Item Bonus + Dexterity Modifier (capped by armor)
The proficiency system is central here. A character Untrained in armor adds +0, Trained adds +2, Expert adds +4, Master adds +6, and Legendary adds +8 — per the Archives of Nethys Second Edition rules. This means a 5th-level character who is Expert in light armor wearing studded leather (+2 item bonus) with a +3 Dexterity modifier has an AC of 24 (10 + 5 + 4 + 2 + 3).
Second Edition also caps how much Dexterity applies based on armor weight. Unarmored characters apply their full Dexterity modifier. Light armor typically caps at +4 or +5. Heavy armor such as full plate caps it at +1. This prevents high-Dexterity characters from making heavy armor redundant and vice versa.
The key dimensions of the Pathfinder system — including the proficiency scale and item bonus structure — shape every armor decision a player makes.
Common scenarios
Three situations come up constantly at the table:
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New character in First Edition wearing no armor: A wizard with 10 Strength and 16 Dexterity has a base AC of 13. Adding a mage armor spell (+4 armor bonus) raises that to 17 — making the spell essentially mandatory for low-level arcane casters who can't wear armor without penalties.
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Fighter in Second Edition choosing between armor types: A Fighter at 1st level Trained in all armor categories will get the same +2 proficiency bonus from leather or full plate. The difference is the item bonus (leather gives +1, full plate gives +6) minus the Dexterity cap. A Fighter with only +1 Dexterity modifier loses almost nothing to heavy armor's cap and gains dramatically from the item bonus.
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Shield use: Both editions treat shields separately from armor. In Second Edition, a character must use the Shield Block reaction to apply the shield's Hardness against incoming damage — shields no longer passively add to AC but instead reduce damage when actively raised, a significant tactical shift from First Edition.
Decision boundaries
The most consequential choice is light vs. heavy armor for martial characters, and it comes down to the Dexterity modifier versus the item bonus differential.
| Armor Category | Typical Item Bonus (2E) | Dex Cap (2E) | Check Penalty |
|---|---|---|---|
| Unarmored | +0 | Unlimited | None |
| Light | +1 to +2 | +4 to +5 | 0 |
| Medium | +3 to +4 | +2 to +3 | −1 to −2 |
| Heavy | +5 to +6 | +0 to +1 | −2 to −3 |
(Source: Archives of Nethys, PF2E Armor)
A Dexterity modifier of +4 or higher generally favors light armor in Second Edition. Below +2, heavy armor produces a higher AC at almost every level. The check penalty matters for characters who use stealth or athletic skills heavily — a rogue in full plate is mechanically possible but faces a −3 penalty on most movement-based skills.
For newer players, the full Pathfinder rules overview explains how AC fits into the broader combat resolution system. The structural logic of how attacks, saves, and defenses relate to each other is covered in the conceptual overview of how the Pathfinder system functions.
In First Edition, stacking rules add complexity: armor bonuses don't stack, deflection bonuses don't stack with other deflection bonuses, and natural armor from multiple sources uses the highest value only. The Archives of Nethys First Edition (aonprd.com) documents all stacking restrictions in the Combat chapter.